(a) Field of the Invention
The present invention relates generally to spacecraft or satellite attitude determination systems and, more particularly, to a method and apparatus for correcting star tracker low spatial frequency errors to improve attitude determination performance in stellar-inertial attitude determination systems.
(b) Description of Related Art
The term attitude is used to describe the orientation of an object with respect to a reference orientation. Attitude is of particular interest in satellite or spacecraft operations. For example, if a satellite is used in a communications application it is necessary that the satellite be oriented in the proper direction to receive and/or transmit relevant information for the communication link.
The attitude of a satellite is determined by computations based on the output of sensors located on the satellite. Gyros and object trackers (such as star trackers, sun sensors, and earth sensors) are two types of sensors that may be used in attitude determination systems. In general, gyros are used to determine the rate at which the spacecraft is moving, and star trackers are traditionally used to provide an absolute reference for spacecraft attitude. By integrating gyro output, spacecraft attitude may be determined dynamically with respect to a known attitude provided by the star trackers. The use of gyros in conjunction with star trackers is commonly referred to as a stellar-inertial attitude determination system.
Object trackers such as star trackers, earth sensors, or sun sensors are used to determine the orientation or attitude of the satellite with respect to the objects being tracked. Star trackers commonly use a CCD array to measure heat or light emitted from the tracked object. Typically, satellites use an ephemeris or star catalog system to determine the location of the objects being tracked with respect to the earth. An ephemeris or star catalog system uses a table containing the coordinates of a celestial body at a number of specific times during a given period. Using ephemeris techniques and object trackers, spacecraft attitude with respect to the earth can be determined.
Star trackers measure the positions of stars in the star tracker field of view (FOV). Several types of errors typically corrupt star tracker position measurements, thereby resulting in attitude determination errors. These star tracker errors can be generally attributed to temporal noise (that changes over time), high spatial frequency (HSF) errors that change rapidly as stars move across the star tracker FOV, and low spatial frequency (LSF) errors that change slowly as stars move across the star tracker FOV.
Temporal noise is typically uncorrelated over time and may be heavily attenuated using a standard six-state Kalman filter, which is generally used in spacecraft attitude determination systems. Similarly, HSF errors exhibit rapid variations, even when the spacecraft moves relatively slowly (e.g., as with satellites in geosynchronous orbit), because HSF errors typically arise from variations between adjacent pixels (i.e., one cycle per pixel) within a CCD-based star tracker, for example. Such HSF errors are also heavily attenuated using a standard six-state Kalman filter.
LSF errors are typically caused by widely known non-ideal star tracker characteristics such as CCD array unevenness, optical deformation, effective focal length degradation and charge transfer efficiency degradation. As stars move across the star tracker FOV the error introduced by LSF errors changes very slowly, particularly if spacecraft moves at a slow rate (e.g., as with a satellite in geosynchronous orbit). Thus, these LSF errors cannot be effectively filtered out or compensated for using the standard six-state Kalman filter because they fall within the operating bandwidth of the filter.